Little is known of pre-Wisconsinian ice advances in Wisconsin. The map shows
ice extending to the outer limits of known pre-Wisconsinian glacial deposits,
and also follows the suggestion of Gene Laberge in Geology of the
Lake Superior Region (Geoscience Press, 1994) that early Pleistocene ice may
have governed the present course of the Mississippi River. However, little
is known of the extent and timing of individual ice advances and nothing at all
is known in areas covered by late Wisconsinian deposits. The area shown as ice
covered was probably never entirely ice-covered at any one time.

How much of the so-called "Driftless Area" was truly never
glaciated is controversial. Some early workers claimed to have found erratics
beneath loess within the conventionally mapped Driftless Area.